worktree: add --[no-]track option to the add subcommand

worktree: add --[no-]track option to the add subcommand

Currently 'git worktree add' sets up tracking branches if '<branch>' is
a remote tracking branch, and doesn't set them up otherwise, as is the
default for 'git branch'.

This may or may not be what the user wants.  Allow overriding this
behaviour with a --[no-]track flag that gets passed through to 'git
branch'.

We already respect branch.autoSetupMerge, as 'git worktree' just calls
'git branch' internally.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>